Turbine facility and water treatment method for heater drainage water

ABSTRACT

Provided are a turbine facility, in which iron oxide particle scale that adheres to inner surfaces of boiler tubes and impedes heat transfer can be efficiently removed from heater drainage water; and a water treatment method for heater drainage water in the turbine facility. The turbine facility includes a boiler  9 , steam turbines  12  and  16 , a condenser  1 , feedwater heaters  5  and  8  which are interposed in water supply lines  4  and  6  that supply condensate condensed by the condenser  1  to the boiler  9 , and in which part of steam supplied from the steam turbine  12  to a repeater is extracted as extraction steam, and the feedwater is heated using the extraction steam, and a filtration device  19  in which heater drainage water discharged from the low-pressure feedwater heater  5  is filtered and supplied to the water supply system for recovery. The filtration device  19  includes a filter having a pore size of 1 to 5 μm.

FIELD OF INVENTION

The present invention relates to a turbine facility, and more particularly relates to a turbine facility equipped with a mechanism that filters heater drainage water and recovers water to a feed pipe. Furthermore, the present invention relates to a water treatment method for heater drainage water in the turbine facility.

BACKGROUND OF INVENTION

In thermal and nuclear power plants and the like, generated high-temperature, high-pressure steam is supplied to a turbine, and the turbine is driven by the steam to generate power. The steam which has driven the turbine is cooled and converted to the form of water by a condenser, and then the water is heated again and supplied to a boiler, nuclear reactor, or steam generator for reuse.

In large-scale power generation facilities, high-pressure and low-pressure straight multi-stage steam turbines are used in many cases. The turbine is rotated by high-temperature, high-pressure steam generated in a boiler or steam generator, and thus a power generator is rotated. As steam expands, its enthalpy decreases and the steam becomes wet steam. In the state of wet steam, the energy conversion efficiency in the turbine decreases, and therefore, partial of wet steam is performed at a predetermined stage of the turbine. The extraction steam has a large amount of energy including latent heat of vaporization. Accordingly, for the purpose of heat recovery, the bleed of steam from the predetermined stage of the turbine is led to a heat exchanger and subjected to indirect heat exchange with condensate, thus heating the condensate. A heat exchanger which heats the condensate using the extraction steam from a high-pressure turbine is referred to as a “high-pressure heater”, and a heat exchanger which heats the condensate using the extraction steam from a low-pressure turbine is referred to as a “low-pressure heater”.

The extraction steam from the low-pressure turbine is low in temperature and pressure compared with the extraction steam from the high-pressure turbine. Therefore, the condensate discharged from a condenser passes through a low-pressure heater first, then passes through a deaerator, a high-pressure heater, and an economizer, and circulated again as feedwater to the boiler. Furthermore, high-pressure heater drainage generated by condensation in the high-pressure heater and low-pressure heater drainage generated by condensation in the low-pressure heater are led to a condensate main pipe, and recycled as boiler feedwater.

In boilers, water quality management of feedwater is important in order to prevent damage on heat transmission tubes due to corrosion. Hitherto, for the purpose of maintaining the pH of boiler feedwater on the alkali side, volatile amines and nitrogen compounds, such as hydrazine and ammonia, have been used. Furthermore, these pH adjustors also act as reducing agents and form a black oxide layer of magnetite (Fe₃O₄) on the boiler tube surface, thus exhibiting anti-corrosion behavior. Such a boiler water treatment method is referred to as “AVT (All Volatile Treatment)” and has long been considered as the standard for boiler water quality management.

As the thickness of the magnetite layer increases excessively, the heat-transfer coefficient decreases. Furthermore, magnetite forms a wavelike oxide layer on the boiler tube surface and increases the water flow resistance of boiler water, resulting in a decrease in comprehensive energy conversion efficiency. Therefore, in power generation facilities, once in three to four years, chemical cleaning is performed during the periodic maintenance so that excessive growth of magnetite oxide layers can be controlled and corrosion prevention of boiler tubes and decreases in resistance of heat transfer and water flow resistance can be achieved.

For about 20 years, a boiler water quality management technique referred to as “CWT (Combined Water Treatment)” has been prevalent mainly in Europe and North America. In this method, feedwater including both condensate and makeup water is treated with a deaerator, in which oxygen, inert gases, and the like are removed, and then by adding pure oxygen, the oxygen concentration in the feedwater is controlled to about 5 ppb. In the initial phase of transition to CWT, combined treatment using ammonia together with oxygen was mainly carried out. In recent years, oxygen treatment in which oxygen only is added has become the mainstream. By the oxygen treatment, a layer of hematite (Fe₂O₃), which is more oxidized than magnetite, is formed on the boiler tube surface. The hematite layer is very dense, the surface thereof is smoother than that of the magnetite layer, and therefore, the hematite layer does not increase water flow resistance. Furthermore, the hematite layer is also chemically stable and has a high anti-corrosion effect. Therefore, CWT less frequently requires chemical cleaning than AVT. For these reasons, the number of boilers to which CWT treatment is applied has been increasing in large-scale thermal power plants in Japan.

As described above, the condensate from the turbine is heated by a feedwater heater which uses the extraction steam as a heat source. The drainage from the feedwater heater joins the condensate and recycled as feedwater.

In the turbine facility in which CWT treatment was carried out, when the total iron concentration in the condensate, the high-pressure heater drainage, and the low-pressure heater drainage was measured, the iron concentration in the low-pressure heater drainage was markedly higher than that of other water. Thus, it became evident that the cause for increasing the iron concentration in the boiler feedwater was the low-pressure heater drainage.

When the low-pressure heater drainage in the turbine facility, in which CWT treatment was carried out, was made to flow through a filter unit in which membrane filters with effective filter pore sizes of 3, 1, 0.45, 0.2, and 0.1 μm were arranged in series, it was found that 90% or more of iron oxide scale were retained by the membrane filter with an effective filter pore size of 3 μm. In the present invention, the pore size of the filter (which may be described as the effective filter pore size) is indicated by the absolute filter pore size that allows particles with a target particle size to be removed at a probability of 99% or more.

When the iron oxide fine particles were observed with an electron microscope, they were found to be acicular crystals having a very high ratio of length to cross-sectional diameter of the particle (shape ratio). The iron oxide fine particles were separated, and form identification was performed by Mossbauer spectrometric analysis. As a result, it was found that composite oxides, such as α-Fe₂O₃, γ-Fe₂O₃, and α-FeOOH were present in 80% or more, which confirmed the formation of acicular crystals.

In the CWT treatment, the oxygen dissolved in feedwater is consumed for oxide layer formation when being passed through boiler tubes, and the dissolved oxygen concentration gradually decreases. High-temperature, high-pressure steam generated in the boiler decreases in temperature and pressure as being expanded in the turbine. In the low-pressure heater, the saturation temperature becomes 130° C. or lower. In the low-pressure heater, since the extraction steam from the low-pressure turbine is condensed, developed turbulent flow occurs in the heater. Therefore, it is believed that a situation arises where a stable hematite layer is difficult to form on the heating surface of the low-pressure heater. Furthermore, since the temperature of the low-pressure heater is lower than that of the boiler tubes, the oxidation reaction rate of the base material the heat transmission tube decreases, and the formation of the hematite oxide layer further becomes difficult. As described above, on the heating surface of the low-pressure heater, there is a situation where, physically and chemically, formation of the hematite layer is unlikely to sufficiently proceed. Accordingly, it is believed that dissolution of iron from the base material (corrosion) proceeds. Such a form of corrosion is known as FAC (Flow Accelerated Corrosion).

Iron oxide fine particles in the low-pressure drainage are believed to be formed because the dissolved iron is subjected to oxidation in the drain bulk and precipitated as hematite or geothite (FeOOH) particles which have a low solubility and which are chemically stable.

Techniques for the purpose of removing iron oxide fine particles in boiler feedwater have been proposed (Patent Literatures 1 to 3).

Patent Literature 1 describes that condensate is filtered with a membrane having a pore size of 0.01 to 0.3 μm. Patent Literature 2 describes that condensate is filtered with a membrane having a pore size of 1 μm. However, Patent Literatures 1 and 2 do not describe filtration treatment of low-pressure heater drainage.

Patent Literature 3 describes a turbine facility configured to filter low-pressure heater drainage and supply water to a water supply system and a water treatment method of heater drainage water in the turbine facility. In Patent Literature 3, when the iron concentration of drainage water exceeds a predetermined concentration, the drainage water is discharged out of the system. Only when the iron concentration is low, iron is removed with a filter and the filtrate is used as part of boiler feedwater. The reason for this is that, since drainage water basically contains fine iron particles that cannot be filtered, except for the case where the iron concentration is equal to or less than the predetermined concentration, the iron content exceeds the allowable limit for boiler feedwater even if filtration treatment is performed. In such a configuration of Patent Literature 3, in addition to the problem that large-scale equipment is required, there are other problems in that the water recovery rate from heater drainage water decreases because drainage water having a high iron content is discharged out of the system, and the amount of discharge water increases.

LIST OF LITERATURES

Patent Literature 1: Japanese Patent Publication 9-206567 A

Patent Literature 2: Japanese Patent Publication 2000-218110 A

Patent Literature 3: Japanese Patent Publication 2008-25922 A

OBJECT AND SUMMARY OF INVENTION

It is an object of the present invention to provide a turbine facility in which iron oxide particle scale that adheres to inner surfaces of boiler tubes and impedes heat transfer can be efficiently removed from heater drainage water, and a water treatment method for heater drainage water in a turbine facility.

A turbine facility according to the present invention includes a boiler in which steam is generated by heat from a heat source, a steam turbine which is driven by the steam of the boiler, a condenser which condenses steam from the steam turbine, a water supply system which supplies condensate condensed by the condenser as feedwater to the boiler side, a feedwater heater which is interposed in the water supply system and in which part of steam supplied from the steam turbine to a reheater is extracted as extraction steam, and the feedwater is heated using the extraction steam, and a filtration device in which heater drainage water discharged from the feedwater heater is filtered and supplied to the water supply system for recovery, in which the filtration device includes a filter having a pore size of 1 to 5 μm.

A water treatment method for heater drainage water in a turbine facility according to the present invention includes vaporizing and superheating feedwater in a boiler by heat from a heat source, driving a steam turbine by means of generated steam, condensing steam discharged from the steam turbine with a condenser to form feedwater, supplying the feedwater to the boiler side, heating the feedwater in a feedwater heater using extraction steam extracted from part of steam supplied from the steam turbine to a reheater, and filtering heater drainage water which is generated by cooling the extraction steam in the feedwater heater so as to be recovered to a water supply system, in which the heater drainage water is filtered with a filter having a pore size of 1 to 5 μm.

In the present invention, preferably, the total amount of heater drainage water is filtered and supplied to the water supply system. The feedwater heater for filtering drainage water is preferably a low-pressure feedwater heater.

ADVANTAGEOUS EFFECTS OF INVENTION

In the present invention, since iron oxide fine particles are efficiently removed from heater drainage water by filtering the heater drainage water using a filter having a pore size of 1 to 5 μm, adhesion of iron oxide fine particles to inner surfaces of boiler tubes can be prevented.

In the present invention, there is no need for a mechanism to measure the iron concentration in heater drainage water and accordingly change the destination to which heater drainage water is supplied.

In the present invention, the total amount of heater drainage water can be filtered and supplied to the water supply system, and thus the water recovery rate is high.

Most of the iron oxide fine particles introduced into boiler feedwater are attributed to low-pressure heater drainage. In general, a filter has an appropriate flow velocity for use. When low-pressure heater drainage is subjected to filtration treatment, the amount of treated water is about one tenth compared with the case where the total amount of condensate is subjected to filtration treatment. Consequently, it is possible to provide a compact filtration device which has a small number of filters installed.

Many of the iron oxide fine particles generated in the low-pressure heater are acicular crystals that can be retained by a membrane with an effective filter pore size of 3 μm. Therefore, by using a filter with an effective filter pore size of 1 to 5 μm, the particles can be retained sufficiently. Since the filter pore size is large at 1 to 5 μm and the shape of fine particles is acicular, the flow pressure loss is unlikely to increase even when continuously used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a turbine facility according to an embodiment.

FIG. 2 is a graph showing experimental results.

DESCRIPTION OF EMBODIMENTS

The present invention will be described in more detail below with reference to the drawings.

FIG. 1 shows a turbine facility according to an embodiment. Water (condensate and makeup water) in a condenser 1 is supplied through an electromagnetic filter 2 and a deionizer 3 including ion exchange resins, via a line 4, to low-pressure feedwater heaters 5, and heated. The heated water is supplied via line 6 to a deaerator 7, subjected to deaeration treatment, then heated by high-pressure feedwater heaters 8, and supplied to a boiler 9. Steam generated in the boiler 9 is superheated by a superheater 10, and then supplied via a steam line 11 to a high-pressure turbine 12.

Steam flowing out of the high-pressure turbine 12 is sent via a steam line 13 to a reheater 14, reheated, and then supplied via a steam line 15 to a low-pressure turbine 16. The effluent steam therefrom is returned to the condenser 1.

An extraction steam line 17 branches off from the steam line 13. Part of steam is separated from the line 11, supplied to the heat source side of the low-pressure feedwater heater 5, and heat-exchanged with water to form drainage water (low-pressure heater drainage water). The low-pressure heater drainage water is supplied via a line 18 to a filtration device 19, and after being filtered, supplied via a return line 20 to the water side of the low-pressure feedwater heater 5. The return line 20 may be connected to the line 4 on the inflow side of the low-pressure feedwater heater 5 or the line 6 on the outflow side.

The filter used in the filtration device 19 has a pore size (effective filter pore size) of 1 to 5 μm, preferably 1 to 4 μm, more preferably 2 to 4 μm, and still more preferably 2 to 3 μm. When the pore size of the filter is less than 1 μm, the flow pressure loss increases. When the pore size is more than 5 μm, retention of iron oxide fine particles becomes insufficient. The LV of the filtration device 19 is 0.2 to 1.2 m/Hr, and particularly preferably about 0.3 to 1.0 m/Hr.

The material for the filter is not particularly limited. However, since the temperature of low-pressure heater drainage water is 80° C. to 130° C., the material is preferably endurable for use in this temperature range for a minimum of one year. Specifically, a nonwoven fabric composed of polyphenylene sulfide fibers or fluororesin fibers is suitably used. When a nonwoven fabric filter alone is used, deposition of the filter cake and flow of filter fluid may cause distortion of the fiber layer, and the predetermined filtration efficiency may not be obtained in some cases. Therefore, the filter to be used preferably has a three-layer structure in which a nonwoven fabric is sandwiched at both surfaces between spunbonded sheets having a mechanical strength, and these layers are integrated by embossing.

According to this embodiment, since iron oxide fine particles are sufficiently removed from low-temperature heater drainage water, adhesion of iron oxide fine particles to inner surfaces of boiler tubes can be prevented (which also includes suppression). Since the total amount of low-pressure heater drainage water is filtered, the water recovery rate is high, and the configuration of supplying water to the filtration device 19 is simple and low cost.

EXAMPLES Experimental Example 1

Low-pressure heater drainage in a turbine facility of a thermal power plant, in which CWT treatment was carried out, was made to flow through a filter unit, in which first to fifth membrane filters with effective filter pore sizes of 3, 1, 0.45, 0.2, and 0.1 μm were arranged in series, from the 3-μm membrane side at a flow linear velocity (LV) of 2.3 cm/min for 4 Hr. The distribution of the amount of iron oxide retained by the filters with the respective pore sizes was measured. The result thereof is shown in Table 1.

TABLE 1 Weight percentage Filter of total iron (effective filter pore size) retained (%) First membrane filter (3 μm) 95.3 Second membrane filter (1 μm) 1.64 Third membrane filter (0.45 μm) 0.82 Fourth membrane filter (0.2 μm) 1.31 Fifth membrane filter (0.1 μm) 0.95

The sum total of the amount of iron oxide retained by the first to fifth membrane filters was divided by the integrated flow rate and converted into the amount of Fe (iron). The calculation result was 25 μg-Fe/L. The total iron concentration in the filtrate passed through all of the first to fifth membrane filters was 1.4 μg-Fe/L.

Experimental Example 2

Boiler drainage at 125° C. (pressure 0.25 MPa (G)) was made to flow at 580 mL/min through a pleated filter (effective filter pore size: 2 μm) with a diameter of 70 mm and an effective length of the filter surface of 25 mm, which was produced by folding three SMS sheets, each being obtained by sandwiching a nonwoven fabric composed of polyphenylene sulfide thin filaments spun by a melt blow method between spunbonded sheets, followed by embossing. The total iron concentration of the influent water was 48 μg-Fe/L, and the total iron concentration in the filtrate at the outlet of the pleated filter was 2.0 μg-Fe/L.

The particle size distribution of the filter cake obtained by continuously passing water was measured by an ultrasonic particle size analyzer. As a result, as shown in FIG. 2, the 50% by weight average particle size was 7 to 8 μm. The cumulative content of particles having a particle size of 1 μm or less was about 5% by weight, and the cumulative content of particles having a particle size of 5 μm or less was about 40% by weight. This shows that even when a filter with an effective filter pore size of less than 1 μm is used, the particle retention rate is not improved, and that when a filter with an effective filter pore size of more than 5 μm is used, the particle retention rate decreases.

Furthermore, it has become evident that, in this state, even if water passing is continued for 120 days, the differential pressure is about 5 kPa, and even when drainage having a concentration of about 20 μg-Fe/L is made to pass through the filter for one year, the differential pressure does not increase to such an extent that passing of water is impeded.

Although the present invention have been described in detail on the basis of specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

This application claims the benefit of Japanese Patent Application No. 2012-043802, filed Feb. 29, 2012, which is hereby incorporated by reference herein in its entirety. 

1. A turbine facility comprising: a boiler in which steam is generated by heat from a heat source; a steam turbine which is driven by the steam of the boiler; a condenser which condenses steam from the steam turbine; a water supply system which supplies condensate condensed by the condenser as feedwater to the boiler side; a feedwater heater which is interposed in the water supply system and in which part of steam supplied from the steam turbine to a reheater is extracted as extraction steam, and the feedwater is heated using the extraction steam; and a filtration device in which heater drainage water discharged from the feedwater heater is filtered and supplied to the water supply system for recovery, characterized in that the filtration device includes a filter having a pore size of 1 to 5 μm.
 2. The turbine facility according to claim 1, characterized in that, in the filtration device, the total amount of the heater drainage water is filtered and supplied to the water supply system.
 3. The turbine facility according to claim 1, characterized in that the heater drainage water is low-pressure heater drainage water.
 4. A water treatment method for heater drainage water in a turbine facility comprising: vaporizing and superheating feedwater in a boiler by heat from a heat source; driving a steam turbine by means of generated steam; condensing steam discharged from the steam turbine with a condenser to form feedwater; supplying the feedwater to the boiler side; heating the feedwater in a feedwater heater using extraction steam extracted from part of steam supplied from the steam turbine to a reheater; and filtering heater drainage water which is generated by cooling the extraction steam in the feedwater heater so as to be recovered to a water supply system, characterized in that the heater drainage water is filtered with a filter having a pore size of 1 to 5 μm.
 5. The water treatment method for heater drainage water in a turbine facility according to claim 4, characterized in that the total amount of the heater drainage water is filtered with the filter and recovered to the water supply system.
 6. The water treatment method for heater drainage water in a turbine facility according to claim 4, characterized in that the heater drainage water is low-pressure heater drainage water. 